Device for electrical characterization of molecules using CNT-nanoparticle-molecule-nanoparticle-CNT structure

ABSTRACT

A method of forming an electrode is disclosed. A carbon nanotube is deposited on a substrate. A section of the carbon nanotube is removed to form at least one exposed end defining a first gap. A metal is deposited at the at least one exposed end to form the electrode that defines a second gap.

BACKGROUND

The present invention relates to electronic nanodevices, and more specifically, to a device for conducting electricity through a single molecule and methods of fabricating the device.

The current state of nanotechnology has allowed for the possibility of manufacturing electronic devices for applications on a molecular scale, including for example, devices for characterizing electrical properties of single molecules and devices that use single molecules in electronic applications. In exemplary molecular-scale devices, molecules may be inserted into a gap between ends of carbon nanotube (CNT) segments and coupled to the CNT ends to complete an electrical circuit. One of the primary difficulties in manufacturing these devices on the molecular scale is controlling the formation of the molecular-sized gaps between the ends of the CNT segments, often on the order of 1 to 10 nanometers (nm). One method uses lithography to pattern and etch a gap from a single carbon nanotube. However, lithographic methods are generally unstable at this length scale (less than 10 nm) and tend to produce gaps at random locations along the CNT and of varying gap sizes. As a result, lithography and etching on this length scale produces only a small yield of functioning devices.

SUMMARY

According to one embodiment of the present invention, a method of forming an electrode includes: depositing a carbon nanotube on a substrate; removing a section of the carbon nanotube to form at least one exposed end defining a first gap; and depositing a metal at the at least one exposed end to form the electrode that defines a second gap.

According to another embodiment of the present invention, a method forming a molecular dock includes: depositing a carbon nanotube on a substrate; removing a section of the carbon nanotube to form exposed ends of the carbon nanotube, wherein the exposed ends define a first gap; and depositing a metal at the exposed ends of the carbon nanotube to form the molecular dock having a second gap defined by the deposited metal.

According to another embodiment of the present invention, a method of controlling a separation between electrodes includes removing a section of a carbon nanotube to produce a first gap having a first gap length; and depositing a metal of a selected length in the first gap to control formation of the electrodes separated by a second gap having a selected length.

Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 shows a carbon nanotube disposed on a substrate and extending between two contacts;

FIG. 2 shows a patterned resist disposed on the substrate and carbon nanotube of FIG. 1;

FIG. 3 shows carbon nanotube segments resulting from etching the carbon nanotube;

FIG. 4 shows a device that includes metal deposited on the formed ends of respective carbon nanotube segments to create electrodes; and

FIG. 5 shows a flowchart illustrating an exemplary method of manufacturing the exemplary molecular electronic device of the present invention.

DETAILED DESCRIPTION

Disclosed herein is a method of producing a device that includes a carbon nanotube (CNT) having a gap formed therein. In various embodiments, the length or size of the gap is a consistently reproducibly in a sub-10 nm length scale. The present invention embodiments produce the sub-10 nm gap in several stages. First, a first gap is formed in the CNT channel using lithographic and etching processes. The length or size of the first gap is greater than a lower limit of lithography and etching processes. Lithography and etching generally produces gaps with highly-controlled lengths or sizes and locations when the gap size is above this lower limit, but produces gaps of inconsistent lengths/sizes and location when the gap size is less than this lower limit. Once the first gap is formed, a metal may be deposited in the first gap to create a second gap defined by a separation between the metals deposited in the first gap. The length of the metals is generally a known value and as a result the length of the metals in the first gap is controllable. Thus, the length of the second gap, which is a composite of the length or size of the first gap and the length or size of the metal, may be controlled. The metal deposited in the first gap form electrodes, which may be used in molecular-sized devices. In various embodiments, the size of the first gap may be selected or adjusted to produce a desired length of the second gap.

FIGS. 1-4 show a manufacturing process for creating CNT electrodes according to an exemplary embodiment of the present invention. FIG. 1 shows a carbon nanotube 102 disposed on a substrate 104 and extending between two contacts 106 a and 106 b. One end of the carbon nanotube 104 is electrically coupled to the first contact 106 a and an opposing end of the carbon nanotube is electrically coupled to the second contact 106 b. Then carbon nanotube 102 may therefore be used to conduct electricity between the contacts 106 a and 106 b. In an exemplary embodiment, the carbon nanotube 102 forms a substantially straight line between the first contact 106 a and the second contact 106 b. A distance between first contact 106 a and second contact 106 b may be on the order of hundreds of nanometers in an exemplary embodiment.

FIG. 2 shows a patterned resist 202 disposed on the substrate and CNT of FIG. 1. The resist 202 may be any type of resist used in lithography and etching. The resist 202 is deposited on the carbon nanotube 102, substrate 104 and contacts 106 a and 106 b to a selected depth. A window 204 may then be formed in the resist 202 to expose a section of the CNT 102. The window 202 may be formed by lithographically transferring a window pattern to the resist and removing the resist that receives the window pattern. The length or size of the window 202 is selected to be large enough that the lithographic process is able to produce the window with an acceptable control over the length or size of the window. In various embodiments, the length or size of the window is greater than about 10 nanometers (nm).

FIG. 3 shows carbon nanotube segments 102 a and 102 b resulting from etching the carbon nanotube 102. Etchant is introduced into the window 202 formed in FIG. 2 in order to etch away the exposed section of the CNT 102 exposed by the window 202. In an exemplary embodiment, the etchant may be oxygen plasma. Once the exposed section is etched away and the resist is removed, the two carbon nanotube segments 102 a and 102 b remain. The carbon nanotube segments 102 a and 102 b are substantially collinear and have exposed ends 302 a and 302 b respectively that are formed by the etching process. The exposed ends 302 a and 302 b are opposed to each other across formed gap 305, referred to herein as the first gap. The first gap 305 has a length d₁ and is a defined by the distance between the exposed ends 302 a and 302 b. In various embodiments, the length d₁ of the first gap 305 is greater than about 10 nm.

FIG. 4 shows a device 400 that includes metal deposited on the formed ends 302 a and 302 b of respective carbon nanotube segments 102 a and 102 b to create electrodes 402 a and 402 b. In an exemplary embodiment, the metal may be deposited using electroless deposition process. The substrate of FIG. 3 may be immersed in a solution containing selected metal ions that have a redox potential higher than the redox potential of the CNT segments 102 a and 102 b. In an exemplary embodiment, the solution may include PdCl₂ in HCl acid solution which contains Pd²⁺ ions. In alternate embodiments, the solution may include Pd, Pt and Au as well as other metal that may be suitable for deposition. Since the metal ions have a redox potential higher than the redox potential of the CNT segments 102 a and 102 b, the metal ions reduce at the opened ends 302 a and 302 b of the CNT segments 102 a and 102 b to form electrodes 402 a and 402 b at the formed ends 302 a and 302 b. The metal ions may grow at the opened ends 302 a and 302 b up until a saturation point or until an electrochemical equilibrium is reached. The length of the metals deposited is known. Therefore, the length of the electrodes 402 a and 402 b may be controlled. The electrodes 402 a and 402 b form a second gap 405 having a length or size d₂. The length or size of the second gap 405 depends of the length or size of the first gap 305 and the length of the electrodes 402 a and 402 b. In an exemplary embodiment, the length of at least one of the electrodes 402 a and 402 b is a diameter of a single metal ion. In an alternate embodiment, the length of the electrodes 402 a and 402 b may be controlled by controlling the rate of ion growth at the ends 302 a and 302 b. Knowing a rate of ion growth and/or a concentration of the solution, the deposition may be stopped at a selected time to form electrodes 402 a and 402 b of a selected length. In various embodiments, the electrodes 402 a and 402 b may be used to provide a molecular dock for reception of a single molecule. The molecule may then complete an electrical connection between the contacts 106 a and 106 b. Electrical properties of the molecule may then be tested. Alternatively, the electrical properties of the molecule may be used to perform a function. Additional functions of the electrodes 402 a and 402 b besides those specifically disclosed herein are within the scope of the invention.

FIG. 5 shows a flowchart illustrating an exemplary method of manufacturing the exemplary molecular electronic device of the present invention. In block 502, a carbon nanotube 102 is deposited in a substrate 104 between two contacts 106 a and 106 b to form a substantially linear carbon nanotube channel. In block 504, a first gap is formed in the carbon nanotube channel using lithography and etching methods. The carbon nanotube channel is thus segmented into a first carbon nanotube segment 102 a and a second carbon nanotube segment 102 b separated by the first gap 305. The first gap is large enough where lithography and etching can produce a gap of suitably controllable size and location. In block 506, a second gap is formed in the first gap by electroless deposition of metal in the first gap. The length of the metal that is deposited in the first gap is a known value. In various embodiments, the length of the metal is a length of a single metal ion. Since the second gap is formed by two processes that are operated over length scales over which length or size may be substantially controlled, (i.e., greater than about 10 nm for lithography and etching of the first gap and less than about 10 nm for electroless deposition to form the second gap) the length of the second gap is controlled to a selected precision. As an illustrative example, the first gap may be etched in the nanotube to 20 nm (+/−2 nm). Nanoparticles that are 7 nm in diameters may be deposited in the first gap using electroless deposition. The resulting second gap is therefore about 6 nm (+/−2 nm). The length or size of the second gap is below the lower length or size limit for which lithography and etching may control the length/size and location of the gap.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated

The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.

While the exemplary embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described. 

What is claimed is:
 1. A method of forming electrodes, comprising: depositing a carbon nanotube on a substrate; removing a section of the carbon nanotube to form a first carbon nanotube segment having a first exposed end and a second carbon nanotube segment having a second exposed end, wherein the exposed end of the first carbon nanotube segment and the exposed end of the second carbon nanotube segment define a first gap; and depositing a metal at the exposed end of the first carbon nanotube segment to form a first electrode and at the exposed end of the second carbon nanotube to form a second electrode, wherein the first electrode and the second electrode define a second gap.
 2. The method of claim 1, wherein depositing the metal further comprises electroless deposition of the metal at the at least one of the first exposed end and the second exposed end.
 3. The method of claim 1 further comprising controlling at least one of: (i) a length of the deposited metal; and (ii) a rate of ion growth at the first exposed end and the second exposed end in order to obtain the second gap having a selected length.
 4. The method of claim 1, wherein removing the section of the carbon nanotube further comprises performing lithography forming a window in a resist layer formed on the carbon nanotube, wherein a dimension of the window is about a length of the first gap.
 5. The method of claim 1, wherein a length of the first gap is greater than about 10 nanometers.
 6. The method of claim 1, wherein a length of the second gap is less than about 10 nanometers.
 7. A method of forming a molecular dock, comprising: depositing a carbon nanotube on a substrate; removing a section of the carbon nanotube to form a first carbon nanotube segment having a first exposed end and a second carbon nanotube segment having a second exposed end, wherein the first exposed end and the second exposed end define a first gap; depositing a metal at the exposed end of the first carbon nanotube segment to form a first electrode; and depositing a metal at the exposed end of the second carbon nanotube segment to form a second electrode, wherein the first electrode and the second electrode define a second gap that forms the molecular dock.
 8. The method of claim 7, wherein depositing the metal further comprises performing electroless deposition of the metal at the first exposed end and second exposed end.
 9. The method of claim 7 further comprising selecting a length of the first gap and at least one of: (i) a length of the deposited metal; and (ii) a rate of ion growth at the first exposed end and the second exposed end in order to form the molecular dock to have the second gap of a selected length.
 10. The method of claim 7, wherein removing the section of the carbon nanotube further comprises etching the carbon nanotube over a length scale over which etching is a substantially controllable process.
 11. The method of claim 10, wherein a length of the first gap is greater than about 10 nanometers.
 12. The method of claim 7, wherein a length of the second gap is less than about 10 nanometers.
 13. A method of controlling a separation between carbon nanotube electrodes, comprising: removing a section of a carbon nanotube to produce a first carbon nanotube segment having a first exposed end and a second carbon nanotube segment having a second exposed end, wherein the exposed end of the first carbon nanotube segment and the exposed end of the second carbon nanotube segment define a first gap having a first gap length; and depositing a metal of a selected length at the exposed end of the first carbon nanotube segment to form a first electrode and at the exposed end of the second carbon nanotube segment to form a second electrode in order to form a second gap separating the first second electrode and the second electrode.
 14. The method of claim 13, wherein depositing the metal in the first gap further comprises performing electroless deposition of the metal at the first exposed end and the second exposed end.
 15. The method of claim 13, wherein removing the section of the carbon nanotube further comprises etching the section of the carbon nanotube over a selected length over which etching is a controllable process.
 16. The method of claim 13, wherein a length of the first gap is greater than about 8 nanometers and a length of the second gap is less than about 10 nanometers. 